This invention generally relates to optical correlator systems; and more specifically, to multiple channel optical correlator systems.
Optical correlator systems are used to detect the presence of a selected target in a scene or a field of view. In an optical correlator system, a coherent light beam is passed through a view or scene, which may include the selected target, and then transmitted through a matched filter plate. The matched filter plate contains a matched filter, which is a recording of a diffraction pattern, unique to the selected target; and if the selected target is present in the submitted view, the matched filter plate redirects a portion of the beam incident on it into a relatively intense output beam at a predetermined angle relative to the incident beam, and an inverse transform lens brings this output beam from the matched filter plate to a focus. However, if the selected target is not present in the submitted view, any output beam of the matched filter plate at this selected angle is relatively weak and diffused, and this output beam remains relatively diffused as it passes through the inverse transform lens. A light sensitive detector is located in the focal plane of the inverse transform lens; and when light of a sufficient intensity is focused on that detector, an output signal is produced. This output signal may be used to trigger some type of device, which depending on the apparatus in which the target recognition system is used, might be a simple alarm or a complex robotic guidance system, for example.
The capacity of an optical correlator system can be significantly increased by providing the system with a matched filter plate having a multitude of matched filters, and a multiple beam generating holographic lens that produces a matrix of output beams, and by focusing each of these output beams onto a respective one of the matched filters of the matched filter plate. Each of the matched filters employed in the system may be unique to a respective view of one target, or these matched filters may represent plural targets, and a correlator system having a multitude of such matched filters may be used, for example, to detect plural targets in a scene, or to detect one or more targets in a scene independent of the orientation of the target or targets in that scene.
A matched filter consists of a multitude of closely packed diffraction lines or patterns, which can be considered as covering a circular area of defined diameter, with the length of that diameter depending on the wavelength and the Fourier Transform focal length of the construction beam used to form the matched filter, the size of the object presented to that construction beam, and the percentage of maximum correlation that an operator wants to be able to obtain. Typically, these matched filters are arranged in a matched filter plate in a regular geometrical array. This arrangement, though, leaves a considerable amount of space on the matched filter plate unused, reducing the total number of matched filters that can be recorded on any given matched filter plate.